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Appendix VL

1069

rise of the pencil being proportionate to the travel of the jockey
weight, transmitted by water through pipe/, and governed by the
valves gg. The rotation of d indicates the deformation, being
actuated by the pull of the wire e.

Fig. 951 shews the various straining heads as arranged for the
tensile, compressive, shear, and deflection tests, the letters D, K, z,
and L corresponding with Fig. 950. For the shear test a plunger
b is fixed to the main ram D and enters a socket a attached to the
head K. Each of these carries a knife or shearing surface, and
the two are compelled to slide closely by the action of the roller */,
the specimen being meanwhile wedged in the socket a. Tension
sockets e e are supplied to" the heads K and z, and the hemispheres
are adjusted to true axial line by the set screws in the fixed ring/.
Compression plattens gg are similarly placed in z and L, also
adjustable on spherical surfaces by the screws h //, and the screws
jj serve to shoot out the square bolts for fixing the head z, as
before mentioned. For deflection tests the double girder k is laid
across the bed, the thrust being received on the head L. A
special socket m is put in the head z to receive the pressure
foot ?i, which is supplied with two half cylinders to distribute the
pressure and avoid indenting; and the specimen beam / is sup-
ported on loose cylinders contained by the brackets //.

For torsion tests an apparatus is provided which is essentially
apart from the machine, consisting of a worm gear not unlike
Fig- 336, p. 379-

P. 384. Strain Measuring.—All instruments that are devised
for the purpose of measuring the minute deformations within the
elastic limit of test specimens are termed Extensometers^ whatever
the nature of the strain that is being recorded. They naturally
divide themselves into two classes—viz., those where the magnifi-
cation is performed entirely by mechanical means, and those
where the agency of optics is more or less invoked. Under the
former are included all forms of lever extensometer, as well as
the screw micrometer ^illustrated by "Unwinds apparatus on p. 382,
the accuracy of these being generally about one io,oooth of an
inch. Under the optical ckss may be mentioned Ewing's extenso-
meter, where the mechanical magnification is 2:1, after which the